Typically an engine combusts fuel in a combustion chamber formed by the cylinder, cylinder head, and piston that reciprocates in the cylinder. Explosion force from the combustion rotates a crankshaft through a connecting rod that is connected to the piston and, thereby, converts thermal energy into mechanical energy. The combustion chamber is mounted with an intake pipe to provide the air-fuel mixture, and an exhaust pipe to discharge burned gas. The intake pipe is typically installed with a throttle valve and airflow sensor.
An Electronic Control Unit (ECU) is programmed to decide, according to an amount of air detected via an airflow sensor, an appropriate amount of fuel for activating the engine. However, if vibration and reverse-flow are generated in the intake pipe of the engine, the output value of the airflow sensor can be inaccurate. Therefore, the airflow sensor should first be diagnosed as to whether it is in a reliable state, and if not, the fuel amount should be determined regardless of the air amount measured via the airflow sensor.